Robotic parts including wrist mechanisms are designed to orient the robot's end effector as demanded by workpiece placement. Wrist mechanisms provide three articulations that offer motions commonly called pitch, yaw and roll. Similar articulations of the arm deliver the wrist assembly anywhere in the robot's sphere of influence. Thereafter it requires three more articulations of the wrist mechanism for universal orientation of the end effector.
One class of conventional prior art wrist mechanisms has a wrist structure mounted on a rotating element which constitutes one of the wrist axes of movement (i.e the roll axis). Pitch and yaw axes are typically built up on the rotating element and are usually mutually orthogonal. One of the pitch and yaw axes (i.e. a first pitch axis) is perpendicular to the roll axis. The other of the yaw and pitch axes (i.e. a second pitch axis) is usually perpendicular to the first pitch axis and may or may not intersect the roll axis. This class of wrist is usually known as a roll-pitch-pitch-wrist (i.e. RPP wrist).
Some RPP wrists have all axes intersecting at a single point as disclosed in U.S. Pat. Nos. 3,784,031, 4,068,536, 4,402,234, 4,499,790, 4,574,655, 4,594,918 and 4,608,884. Other RPP wrists have pairs of intersecting axes with two points for axes intersection that are offset from each other as described in U.S. Pat. Nos. 3,826,383 and 4,030,617. Patent application Ser. No. 928,745, filed Nov. 10, 1986 and having the same Assignee as the present application also discloses such a wrist.
RPP wrists usually require the use of gears for the transmission of motion from stationary actuators to the second and third pitch axes across the interface of the bearing elements of the roll axis. One disadvantage of such an arrangement is the kinematic interdependence of the three axes that results from mounting the two pitch axes on the roll axis. This interdependence not only complicates the data processing of axes commands but also makes the motion unnatural and difficult to relate to in terms of human wrist movements. This is of particular importance for robots that are taught by the lead-through method with end effectors led by the hand of a human operator. The unnatural feeling of the robot movement generates difficulties in moving the robot by hand. For example, if the required motion is a yaw motion in a horizontal plane, the roll axis has to be oriented such that one of the two pitch axes is vertical. The operator may spend unnecessary time trying to figure out the appropriate orientation and a smooth approach to that orientation.
Most mechanical wrist mechanisms do not provide an unobstructed passageway therethrough to provide protection for hoses and wires which respectively provide fuel and power to a work tool mounted on the free end of the wrist mechanism. U.S. Pat. No. 4,218,166 discloses a multi-axes wrist mechanism which provides independent movement about two axes and which maintains an unobstructed passage through a stationary base to a work tool.
The U.S. Pat. No. 4,402,234 to Malarz et al discloses a wrist mechanism having three independent rotational axes which are mutually intersecting. A gearing mechanism controls the independent rotation of components supported for rotation about the three axes and also provides an unobstructed internal passageway for housing hoses and/or wires. Use of such a wrist mechanism however, is somewhat limited and difficult due to the relatively small internal bend radius of the wrist mechanism. This imposes limitations on the amount of rotational movement that such hoses and wires will permit. Such a small bend radius furthermore makes programming of the robot to the which the wrist mechanism is attached more difficult. This occurs because the freedom of movement of the wrist mechanism is limited, which the programmer must take into consideration.
The above-noted patent application discloses a three-axes wrist mechanism also having an unobstructed passageway and includes three independent rotational axes which have a pair of mutually intersecting points to provide additional flexibility in positioning a wide variety of tool members in a work envelope. The above-noted application is an improvement over the U.S. Pat. No. 4,402,234 in that the wrist mechanism provides a relatively generous internal bend radius to simplify programming of the robot and also offers the use of larger gears so that the wrist mechanism can support and manipulate relatively heavy tools attached thereto.
U.S. Pat. Nos. 4,576,545, 4,353,677, 4,435,120, 4,568,311 and 4,624,621 discloses wrist mechanisms having multiple bends so that the wrist mechanism can span an arcuate length. However, each of the wrist mechanisms requires the use of gears to synchronize and affect a gradual orientation change in the pitch and yaw motions in two or more stages of bend. The use of gears not only complicates the mechanism and increases its cost, but also reduces its power transmission, efficiency, accuracy and reliability.
An early example of an attempt to provide a flexible robot arm is disclosed in the U.S. Pat. No. 4,107,948. The robot arm comprises a plurality of interconnected rigid links which are adapted to be driven for controlled movement by a robot drive mechanism. Successive links are linked together in the fashion of chain links and are mutually pivotally connected by means of universal joints disposed between the interlinking ends of the links. The links define at least a part of the universal joints. A robot arm member has a protruding driven shaft for turning the jointed robot arm about its own axis and is arranged to be driven by a drive ring in sliding contact with the exterior of one of the universal joints. However, the mechanism is constructed so that it is locked at all times and cannot be utilized as described.
A flexible robot ar related to U.S. Pat. No. 4,107,948 noted above, is disclosed in U.K. Patent Application No. 2,127,774. This robot arm corrects the inoperativeness of the U.S. Pat. No. 4,107,948 device by adding a sliding motion to the rotary freedom of joints 11 and 15. The resulting robot arm has: (1) rotary and sliding joints or parts; (2) five rotary joints; (3) two sliding joints; and (4) six linkages.
U.S. Pat. No. 4,531,885 discloses a device for use in a robot which can displaced angularly about orthogonal axes by use of two linear actuators. The device includes a central element supported to the wrist structure on a universal joint. The central element carries an output shaft and may be oriented by rotating a shaft that carries the universal joint. A bearing separates the point of attachment of the center element from the points of attachment of the linear actuators and allows their relative rotation.
The device of the '885 Patent is limited by the use of a single universal joint which limits the angle size by which the central element can be allowed to rotate about the two orthogonal axes. Also, the device is constructed so that a central cavity cannot be constructed to allow for internal routing of service lines to the end effector of the robot. Because the first two axes of wrist motion are bending or pitch motion and the third a roll motion, this type of wrist is usually known as a pitch pitch roll or PPR wrist.
Another prior art robot wrist is similar to the wrist described in the '885 Patent except that the universal joint is replaced by a spherical gear arrangement which allows angular movement about two orthogonal axes as well as the rotation of a central element.